Nano avalanche photodiode architecture for photon detection

ABSTRACT

An integrated circuit includes a substrate material that includes an epitaxial layer, wherein the substrate material and the epitaxial layer form a first semiconductor material with the epitaxial layer having a first conductivity type. At least one nanowire comprising a second semiconductor material having a second conductivity type doped differently than the first conductivity type of the first semiconductor material forms a junction crossing region with the first semiconductor material. The nanowire and the first semiconductor material form an avalanche photodiode (APD) in the junction crossing region to enable single photon detection. In an alternative configuration, the APD is formed as a p-i-n crossing region where n represents an n-type material, i represents an intrinsic layer, and p represents a p-type material.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/356,152, filed 18 Nov. 2016, which is a divisionalapplication of U.S. patent application Ser. No. 14/185,567, filed 20Feb. 2014, now U.S. Pat. No. 9,570,646, issued on 14 Feb. 2017, both ofwhich are incorporated herein in their entirety.

TECHNICAL FIELD

This disclosure relates to semiconductor detectors, and moreparticularly to detectors that employ nanowire configurations for photondetection.

BACKGROUND

There is a need for sensitive detectors that can detect photon energyeven in low-light conditions. Various applications utilize suchdetectors for low light imaging, laser communications applications, andapplications such as LADAR where coherent lasers are employed instead ofradio waves as in conventional RADAR. The carrier frequency of 1 umLADAR is about 3×10¹⁴ Hz, for example, and can provide about a 100,000times improvement in spatial resolution over 1 GHz RADAR, for example.The promise of great improvement in spatial resolution has given impetusto the development of various LADAR applications. In addition to LADAR,other imaging applications also require detectors for very low lightconditions.

In very low light applications, internal detector gain is required toboost the received photon signal above the noise floor of subsequentelectronics and signal processing. For many years, the only device thatprovided such gain was the photomultiplier tube (PMT) based on vacuumtube technology. While offering high gain, the PMT has a number ofpractical limitations. Such limitations included dealing with a bulkyvacuum tube, offering limited linearity, providing a narrow spectralresponse range, and providing a low quantum efficiency (QE) (typically<25%). The PMT also generates heat, requires several thousand volts foroperation, and is not well suited for integration into system on chip(SOC) platforms. Also, long detector readout times are not optimal forfluorescent lifetime measurement. Various solid-state alternativesrequire several hundred volts for operation and have process limitationsthat are not compatible with standard semiconductor processing andintegrated electronics.

SUMMARY

This disclosure relates to integrated circuit photon detectors that arecreated at nanowire crossing regions. In one aspect, an integratedcircuit includes a substrate material that includes an epitaxial layer,wherein the substrate material and the epitaxial layer form a firstsemiconductor material with the epitaxial layer having a firstconductivity type. The substrate can be intrinsic or of the firstconductivity type of the epitaxial layer. At least one nanowirecomprising a second semiconductor material having a second conductivitytype that is different than the first conductivity type of the epitaxiallayer of the first semiconductor material forms a junction crossingregion with the first semiconductor material. The nanowire and the firstsemiconductor material form an avalanche photodiode (APD) in thejunction crossing region to enable single photon detection.

In another aspect, an integrated circuit includes a first nanowirecomprising a first semiconductor material having a first conductivitytype. The first nanowire has an intrinsic layer formed over the firstsemiconductor material to increase photon sensitivity. A second nanowirecomprising a second semiconductor material having a second conductivitytype different than that of the first conductivity type of the firstsemiconductor material of the first nanowire forms a junction crossingregion with the first nanowire. The first nanowire and the secondnanowire form an avalanche photodiode (APD) in the junction crossingregion to enable single photon detection. A substrate material providesa base for the junction crossing region formed by the first nanowire andthe second nanowire.

In yet another aspect, a method of forming an integrated circuit isprovided. The method includes forming a substrate material and formingan epitaxial layer on the substrate material. The substrate material andthe epitaxial layer form a first semiconductor material with theepitaxial layer having a first conductivity type. The substrate can beintrinsic or of the first conductivity type of the epitaxial layer. Themethod includes forming at least one nanowire on the epitaxial layer.The nanowire comprises a second semiconductor material having a secondconductivity type that is different than the first conductivity type ofthe epitaxial layer of the first semiconductor material and forms ajunction crossing region with the first semiconductor material. Thenanowire and the first semiconductor material form an avalanchephotodiode (APD) in the junction crossing region to enable single photondetection.

In still yet another aspect, a method of forming an integrated circuitincludes forming a substrate material to provide a base for a junctioncrossing region for an avalanche photodiode (APD). The method includesforming a first nanowire on the substrate material, where the firstnanowire comprises a first semiconductor material having a firstconductivity type. The method includes forming an intrinsic layer overthe first semiconductor material to increase photon sensitivity. Themethod includes forming a second nanowire over the intrinsic layer, thesecond nanowire comprising a second semiconductor material having asecond conductivity type that is different than that of the firstconductivity type of the first semiconductor material of the firstnanowire. The method includes forming a junction crossing region withthe first nanowire and the intrinsic layer. The first nanowire, theintrinsic layer, and the second nanowire form an avalanche photodiode(APD) in the junction crossing region to enable single photon detection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a nanowire configuration that forms anavalanche photodiode (APD) at nanowire crossing points for detectingphotons at various wavelengths.

FIG. 2 illustrates an example of a crossing region for an avalanchephotodiode (APD) from a top view and a side view.

FIGS. 3 and 4 illustrate alternative examples of integrated circuitconfigurations that form an avalanche photodiode (APD) at nanowirecrossing points for detecting photons at various wavelengths.

FIGS. 5 and 6 illustrate alternative examples of integrated circuitconfigurations that form an avalanche photodiode (APD) at nanowirecrossing points for detecting photons at various wavelengths where Braggmirrors are employed to increase detector sensitivity.

FIG. 7 illustrates a process to form the integrated circuitconfiguration depicted in FIG. 3.

FIG. 8 illustrates a process to form the integrated circuitconfiguration depicted in FIG. 5.

FIG. 9 illustrates a process to form the integrated circuitconfiguration depicted in FIG. 4.

FIGS. 10 and 11 illustrate example diagrams for selecting materials todetect differing wavelengths for a crossed nanowire avalanche photodiode(APD).

FIG. 12 illustrates an integrated circuit configuration wherecrossed-nanowire avalanche photodiodes (APD) are employed to construct apixel and a pixilated sensor array.

DETAILED DESCRIPTION

This disclosure relates to nanowire avalanche photodiode (or nano-APD)configurations employed as photon detectors that support variousapplications. The nano-APD's can operate in very low-light conditions(e.g., single photon detection) with improved bandwidth while mitigatingeffects such as high dark currents that limit the performance ofconventional detector configurations. The nano-APD can be constructedvia semiconductor processing by crossing a semiconductor nanowire of oneconductivity type (e.g., n-type conductivity) over a semiconductornanowire of a different conductivity type (e.g., p-type conductivity) toform PN junctions. At the junctions of the nanowire crossing points ofthe materials, signal amplification can be achieved via the formation ofavalanche photodiodes that can detect received photon energy. By forminga plurality of such crossing points in a given area on a semiconductorsubstrate, a pixel can be formed from the respective crossing pointswhere each crossing point inside the pixel can potentially receive anddetect photon energy. Having multiple crossing points inside of thepixel increases the sensitivity of the pixel detector and furtherimproves signal-to-noise performance over conventional configurations. Aplurality of such pixels can be formed on a semiconductor substrateproviding an integrated circuit photon detector array, for example.

By utilizing crossed nanowires to create the avalanche photodiodes forthe detector, other performance gains can be achieved. For example,nanowires provide reduced resistance and capacitance over conventionalsubstrate signal paths. As such, reduced RC time constants can beachieved that enable a reduction in recovery time for passive quenchingand in gated length for gated passive quenching which in turn lead to anincrease in detector bandwidth and a minimization of dark count rate(DCR). The nano-APD configurations can be formed utilizing differentmaterials that are tailored to the wavelength of the radiation to bedetected. For example, one set of nanowire materials may be selected fordetecting infrared (IR) radiation and another set of nanowire materialsmay be chosen for detecting short wavelength IR (SWIR) radiation, mediumwavelength IR (MWIR) radiation, long wavelength IR (LWIR) radiation, andso forth. Such material selection enables further signal performancegains for a given imaging application.

FIG. 1 illustrates an example of a nanowire configuration that forms anavalanche photodiode (APD) at nanowire crossing points for detectingphotons at various wavelengths. An example region of a semiconductor isshown as a nano-APD crossing area 100 where a semiconductor 110 of afirst material (A) having a first conductivity type (e.g., n-type orp-type) crosses over a semiconductor 120 fabricated of a second material(B) having a second conductivity type that is different than that of thefirst material (e.g., n-type or p-type different than that of the firstmaterial). At the junction of the crossing point of semiconductors 110and 120, an avalanche photodiode (APD) PN junction region 130 is formedwhere light amplification can occur to detect light at variouswavelengths depending on the materials selected for the respectivesemiconductors. In one example, the first and second semiconductormaterial 110 and 120 can be crossed nanowire structures. At the APDjunction region 130, single photon detection can be achieved sinceincreased sensitivity is provided by the nano-scale crossing points.Moreover, in an alternative configuration, nanowires can be grown orplaced directly over an epitaxial layer formed on a substrate to alsoimprove sensitivity and enable single-photon detection.

In some cases, the p-type material can form the bottom nanowire and then-type material can form the top nanowire. In other cases, thesemiconductor types can be reversed (e.g., p-type formed on top andn-type formed on bottom). In yet another example, rather than one of thematerials being a nanowire, the crossing point and junction region 130can be formed by the intersection of a nanowire and a substrate materialhaving an epitaxial layer formed thereon. For example, if the material120 were formed as a substrate material having an epitaxial layer, theAPD junction 130 could be formed when the material 110 was deposited ontop of the substrate 120 as a nanowire. In still yet another example, anintrinsic layer can be grown between the n-type and p-type materials tofurther improve sensitivity. For illustrative purposes, a single APDjunction is described however a plurality of such junctions can beformed in a given area. As will be described and illustrated below, aplurality of APD junctions can be configured in a given unit area toform a pixel for detecting photons. A plurality of pixels can befabricated to form a semiconductor detector for detecting photons.

The electrical transport and optical properties of nanowires are suchthat nano-APDs can be fabricated to achieve high sensitivity, singlephoton detection, and sub-wavelength resolution, for example. This caninclude crossed nanowire array-based nano-APDs which can reduceinter-pixel cross talk. The semiconductor materials can be fabricated byphysical vapor deposition (PVD) and/or chemical vapor deposition (CVD)methods such as used for Ge, CdSe and other materials systems which aredescribed below. The growth and control of the desired nanowire size andorientation can be accomplished by adjusting substrate temperature,fluid flow (nutrients) and the step growth process, for example. Thisprocess enables the fabrication of nanowires ranging from 2-100 nm indiameter and 5 to 250 nm in length, for example. Very high amplificationin crossed nanowire structures can be achieved by employment ofdifferent APD configurations and then optimization of performance byutilizing resonant cavity enhanced APD structures, for example. In onespecific example application, APD configurations allow for therealization of an advanced Laser RADAR (LADAR)-Radio Frequency PowerAmplifier (RFPA). The APD configurations can include: APD detectorarrays fabricated in nanowires and operating in the Geiger mode; APDbias and operating circuits with positive feedback; and multileveldigital logic for minimizing detector pixel size, for example. TheLADAR-RFPA can be a hybrid APD detector array made of Ge, CdS or InGaAsnanowires, for example. The APD configurations can be readout hybridizedwith a pixel readout circuit fabricated in silicon, for example.

The detection of single photons of light via the APD PN junctions 130can enable a wide range of emerging applications includingcommunications, imaging with significant spatial resolution, quantumcryptography and single molecule fluorescence, for example. Thenano-APDs based on at least one of the crossing materials being ananowire can enable single photon detection with high spatialresolution, high multiplication gain, and bandwidth. Alternativeconfigurations will be illustrated and described below based onnanowires involving integration of nanophotonics with nanoelectronicswith application areas ranging from communications and computing toenhanced diagnostics.

As will be illustrated and described below, various integrated circuitconfigurations and material selections can be provided to enhance photondetection along with enabling tuning for reception of photons at variouswavelengths. A first integrated circuit configuration (e.g., See FIG. 3)includes a substrate material (e.g., material 120) that includes anepitaxial layer, wherein the substrate material and the epitaxial layerform a first semiconductor material, and wherein the epitaxial layer hasa first conductivity type. The substrate can be intrinsic or of thefirst conductivity type of the epitaxial layer. At least one nanowire(e.g., material 110) includes a second semiconductor material having asecond conductivity type doped differently than the first conductivitytype of the epitaxial layer of the first semiconductor material andforms the junction crossing region 130 with the first semiconductormaterial. The nanowire and the first semiconductor material form anavalanche photodiode (APD) in the PN junction crossing region 130 toenable single photon detection.

In a second integrated circuit configuration (e.g., See FIG. 4), a firstnanowire (e.g., layer 120) includes a first semiconductor materialhaving a first conductivity type. The first nanowire includes anintrinsic layer formed over the first semiconductor material to increasephoton sensitivity. As used herein, the term intrinsic layer refers to anon-intentionally doped or an un-doped layer that is not associated witha conductivity type such as n-type or p-type. A second nanowire (e.g.,material 110), that includes a second semiconductor material having asecond conductivity type doped differently than the first conductivitytype of the first semiconductor material of the first nanowire, forms ap-i-n junction crossing region 130 with the first nanowire. The crossingof the first nanowire and the second nanowire form an avalanchephotodiode (APD) in the p-i-n junction crossing region to enable singlephoton detection. A substrate material (See FIG. 4) provides a base forthe junction crossing region formed by the first nanowire and the secondnanowire.

The first conductivity type can be a p-type and the second conductivitytype can be an n-type or the first conductivity type can be an n-typeand the second conductivity type can be a p-type, for example. In thefirst configuration, the nanowire is grown on the epitaxial layer viadeposition or grown at a different location than the epitaxial layer andplaced on the epitaxial layer after growth. In the second configuration,nanowires can be grown or placed on the substrate and grown and/orplaced to create the junction crossing regions 130. A Bragg mirror (SeeFIGS. 5 and 6) can be formed over the substrate and below the epitaxiallayer to increase photon detection sensitivity of the APD. At least oneof the first semiconductor material or the second semiconductor materialcan be Si to enable pixel readout of the APD. In another example, theAPD forms a resonant cavity to increase photon detection sensitivity ofthe APD. A plurality of nanowires can form a plurality of junctioncrossing regions 130 in a given area on the epitaxial layer or substratelayer (See FIG. 12A), where the respective junction crossing regionsform an integrated circuit pixel for photon detection. A plurality ofintegrated circuit pixels can be formed on the epitaxial layer orsubstrate layer to form a pixilated sensor array for photon detection(See FIG. 12B). As noted above, the nanowires can be fabricated at about2 to 100 nanometers in diameter and 5 to 250 nanometers in length.

Various material selections can be made to increase APD sensitivity andadjust the wavelengths for APD detection. In one example, at least oneof the first or second semiconductor materials 110 or 120 is Silicon(Si) and at least one of the first or second semiconductor materials isselected from a group consisting of: (Ge), (InGaAs),(InGaAs_(x)P_(1-x)), (CdS), (CdSe), (ZnS), and (ZnSe) to enable photondetection in the near infrared wavelength, where x is a number greaterthan zero and less than or equal to one (including fractions) and where(Ge) is Germanium, (In) is Indium, (Ga) is Gallium, (As) is Arsenic, (P)is Phosphorus, (Cd) is Cadmium, (Se) is Selenium, (Zn) is Zinc, and (S)is Sulfur. In another example, at least one of the first or secondsemiconductor materials 110 or 120 is Si and at least one of the firstor second semiconductor materials is selected from a group consistingof: PbS, PbSe, InSb, GaSb, CdS, CdSe, HgCdSe, HgCdS, and HgCdTe toenable photon detection in the medium or long infrared wavelength, where(Pb) is Lead, (Sb) is Antimony, (Hg) is Mercury, and (Te) is Tellurium.In yet another example, at least one of the first or secondsemiconductor materials 110 or 120 is Si and at least one of the firstor second semiconductor materials is selected from a group consistingof: PbSe_(x)Si_(1-x), Cd_(x)Hg_(1-x)S, Pb_(x)Hg_(1-x)S, CdSe_(x)S_(1-x),As2SxSe5-x, GeSe_(x)S_(y)Te_((1-x-y)), CuAlS_(2-x)Se_(x),As_(x)Se_(1-x)Ge₅, AgGaGe₂—GeSe₂, and AgGaS₂—GeS₂ to enable tunablewavelength characteristics, wherein x and y are numbers greater or equalto zero and less than or equal to one (including fractions) and where(Cu) is Copper, (Al) is Aluminum, and (Ag) is Gold. Silicon can beintrinsic or doped for example with elements including Boron (B),Arsenic (As), Gallium (Ga), or other suitable silicon dopant.

FIG. 2 illustrates an example of a crossing region for an avalanchephotodiode (APD) from a top view and a side view. Referring to the topview, a crossing region 210 shows the intersection of an n-type material220 on top and a p-type material 230 on the bottom. The p-type material230 is connected to anode contacts 234 and 238 while the n-type materialis connected to cathode contacts 244 and 248. As noted above, in otherconfigurations, the p-type material could be on the top and the n-typematerial could be on the bottom. The side view shows incident lightpropagating in the crossing region 210, wherein the intersection of then-type and p-type materials form a nano-scale APD as shown at 250. Thelight is primarily absorbed in the n-type semiconductor in this example,while avalanche multiplication takes place in the nano-scale p-njunction 250. The growth techniques described herein do not require thetwo semiconductor materials to be lattice matched. By choosing thematerials systems described herein, several designs for near IR, MWIRand LWIR can be achieved. For example, nano-APD structures can befabricated for the 1.0 to 1.6 um wavelength range.

The nanowires can be fabricated via deposition techniques wherenanometer beads are deposited that form the cylindrical shapes of thewires. The larger the bead, the greater the diameter of the nanowire.Length of the nanowire can be controlled by moving the depositiondispenser for a given distance. Nanowires can be grown directly on thesubstrates and/or epitaxial layers described herein or they canalternatively be grown at a different location and placed on asubstrate, epi-layer, or over top of another nanowire via nano-scaleplacement equipment. Any suitable technique for depositing nanowires orepitaxial layers can be employed such as metal organic chemical vapordeposition (MOVCD), molecular beam epitaxy (MBE) or other suitabledeposition techniques.

FIGS. 3 and 4 illustrate alternative examples of integrated circuitconfigurations that form an avalanche photodiode (APD) at nanowirecrossing points for detecting photons at various wavelengths. FIG. 3shows a substrate material 300 having an epitaxial layer 310 formedthereon. Epitaxy refers to the deposition of an over layer 310 on acrystalline substrate 300. Epitaxial films may be grown from gaseous orliquid precursors. Since the substrate 300 acts as a seed crystal, thedeposited film may lock into one or more crystallographic orientationswith respect to the substrate crystal. A nanowire 320 is deposited (orplaced) on to the epitaxial layer 310, wherein an avalanche photodiodeis formed at crossing region 330.

FIG. 4 shows an alternative configuration where a substrate material 400forms a base for a crossed nanowire configuration. A bottom nanowire at410 is formed having an inner core 420 of one conductivity type (n orp-type) and having an outer shell 430 formed as an intrinsic layer wherethe outer shell is not intentionally doped to produce either an n-typeor a p-type material. The intrinsic layer can increase the sensitivityof the resultant crossing junction. As the inner core 420 is beingdeposited, the doping process supplying the inner core can be terminatedwhile the overall deposition process for the nanowire 410 continuesleaving an intrinsic layer at the outer shell 430. A nanowire 440 of asecond conductivity type (e.g., n-type or p-type) doped differently thanthe inner core 420 can be deposited or placed over the nanowire 410 toform an APD crossing junction 450.

FIGS. 5 and 6 illustrate alternative examples of integrated circuitconfigurations that form an avalanche photodiode (APD) at nanowirecrossing points for detecting photons at various wavelengths where Braggmirrors are employed to increase detector sensitivity. In the example ofFIG. 5, a Bragg mirror 500 can be deposited on top of a substrate 510and below an epitaxial layer 530 before depositing (or placing) ananowire 540. In the example of FIG. 6, a Bragg mirror 600 can bedeposited over a substrate layer 610 and before subsequent nanowirecrossing junctions are deposited and/or placed.

FIG. 7 illustrates an example process to form the integrated circuitconfiguration depicted in FIG. 3. At 700, a substrate layer is formedvia a suitable deposition method such as chemical vapor deposition, forexample. The substrate is typically formed in silicon although othersubstrate materials are possible. At 710, the process includes formingan epitaxial layer on top of the substrate layer via a suitabledeposition method. At 720, a nanowire is deposited or placed on top ofthe epitaxial layer. When placed, available attractive forces betweenthe nanowire and epitaxial layer can hold the nanowire in place. Aplurality of such nanowires can be deposited or placed on the epitaxiallayer in a similar manner to create pixels and pixilated arrays asillustrated and described below with respect to FIG. 12. The epitaxiallayer is doped as one type of conductive material (n-type or p-type) andthe nanowire is doped as a different type to enable formation of the PNjunction of the avalanche photodiode.

FIG. 8 illustrates a process to form the integrated circuitconfiguration depicted in FIG. 5. At 800, a substrate layer is formedvia a suitable deposition method such as chemical vapor deposition, forexample. The substrate is typically formed in silicon although othersubstrate materials are possible. At 810, the process includes forming aBragg mirror on top of the substrate layer via a suitable depositionmethod. This can include chemical processes to enhance the reflectivesurface of the mirror. At 820, an epitaxial layer is deposited over theBragg mirror. At 830, a nanowire is deposited or placed on top of theepitaxial layer. When placed, available attractive forces between thenanowire and the epitaxial layer can hold the nanowire in place. Aplurality of such nanowires can be deposited or placed on the epitaxiallayer in a similar manner to create pixels and pixilated arrays asillustrated and described below with respect to FIG. 12.

FIG. 9 illustrates a process to form the integrated circuitconfiguration depicted in FIG. 4. At 900, a substrate layer is formedvia a suitable deposition method such as chemical vapor deposition, forexample. Before proceeding, it is noted that a Bragg mirror couldalternatively be deposited on top of the substrate at this stage toincrease detector sensitivity such as shown at FIG. 6. For purposes ofbrevity however, such Bragg mirror deposition will not be shown ordescribed herein. At 910, a first semiconductor material having a firstconductivity type is deposited as a bead having a given width andlength. Before the deposition process is completed however, doping issuspended yet the deposition continues such as shown via the dottedarrow at 920. By discontinuing the doping process, a non-intentionallydoped layer or intrinsic layer is formed over the first semiconductormaterial thus forming a nanowire having a doped inner core and an outershell formed of an intrinsic layer. At 930, a second nanowire having aconductivity type (n-type or p-type) different than that of the innercore of the first semiconductor material is deposited or placed on topof the intrinsic layer. In this example, a nano-APD is thus formedhaving a p-i-n crossing junction where at least one of the materials inthe junctions is p-type, at least one of the materials is n-type, and atleast one of n or p-type materials is covered by an intrinsic layer.

FIGS. 10 and 11 illustrate example diagrams for selecting materials todetect differing wavelengths for a crossed nanowire avalanche photodiode(APD). These diagrams illustrate design options to optimize performanceand assist with achieving single photon detection. Nano-APDs scribedherein have tighter size uniformity and incorporate fewer defects due totheir small size. The former allows for higher breakdown voltageuniformity (compared to micron-sized APDs), which reduces dark countrates. The fewer defects result in lower dark count rates for a nano-APDarray configuration compared to those of micron-sized APD arrays. Inaddition, the capacitances and series resistances of nano-sized APDs arelower than their micron-sized counterparts. The small RC time constantreduces the recovery time for passive quenching and the gated length forgated passive quenching, which are useful for minimizing DRC andincreasing bandwidth. The materials consideration can be based on thewavelength of interest. FIGS. 10 and 11 show exemplary band gaps andoptical absorptions for several materials. The materials selection canbe made for the desired wavelengths as described herein.

FIG. 12 illustrates an integrated circuit configuration wherecrossed-nanowire avalanche photodiodes (APD) are employed to construct apixel 12(a) and a pixilated sensor array 12(b). At 1200, an individualpixel is illustrated having a plurality of crossing junctions formingnano-APD's at each junction At 1210, a plurality of such pixels can forman integrated circuit pixilated array. To maximize sensitivity, multiplenano-APDs can be incorporated into each pixel by crossing multiple pwith n nanowires (or multiple p-i-n crossings). These pixels can beabout 0.1 to 50 μm in size and can include numerous individualnano-APDs. Such pixels could then be integrated into a CCD array withreadout circuitry between the pixels. Alternatively, if the nanowiresare grown on oxide, the readout circuitry can be below the pixels, thusoptimizing the fill factor of the array.

What have been described above are examples. It is, of course, notpossible to describe every conceivable combination of components ormethodologies, but one of ordinary skill in the art will recognize thatmany further combinations and permutations are possible. Accordingly,the disclosure is intended to embrace all such alterations,modifications, and variations that fall within the scope of thisapplication, including the appended claims. As used herein, the term“includes” means includes but not limited to, the term “including” meansincluding but not limited to. The term “based on” means based at leastin part on. Additionally, where the disclosure or claims recite “a,”“an,” “a first,” or “another” element, or the equivalent thereof, itshould be interpreted to include one or more than one such element,neither requiring nor excluding two or more such elements.

What is claimed is:
 1. A method of forming an integrated circuitcomprising: forming a substrate; forming a first nanowire comprising afirst semiconductor material having a first conductivity type on thesubstrate; forming an intrinsic layer over the first semiconductormaterial to increase photon sensitivity; and forming a second nanowireover the intrinsic layer, wherein a portion of the second nanowire ispositioned to overlay a portion of the first nanowire, the secondnanowire comprising a second semiconductor material having a secondconductivity type doped differently than the first conductivity type ofthe first semiconductor material of the first nanowire and forming ajunction crossing region with the first nanowire and the intrinsiclayer, wherein the first nanowire, the intrinsic layer, and the secondnanowire form an avalanche photodiode (APD) in the junction crossingregion to enable single photon detection.
 2. The method of claim 1,wherein forming the first nanowire comprises depositing along thesubstrate in a given direction a plurality nanometer beads of a givenwidth and length relative to each other to connect the plurality ofnanometer beads to form the first nanowire.
 3. The method of claim 1,wherein depositing comprises one of metal organic chemical vapordeposition (MOVCD) and molecular beam epitaxy (MBE).
 4. The method ofclaim 2, wherein forming the second nanowire comprises depositing alonga length of the first nanowire a plurality nanometer beads of a givenwidth and length relative to each other to connect the plurality ofnanometer beads to form the second nanowire.
 5. The method of claim 1,further comprising forming a Bragg mirror over the substrate material toincrease detector sensitivity.
 6. The method of claim 1, wherein atleast one of the first or second semiconductor materials is Silicon (Si)and at least one of the first or second semiconductor materials isselected from a group consisting of: Ge, InGaAs, InGaAs_(x)P_(1-x), CdS,CdSe ZnS, and ZnSe to enable photon detection in the near infraredwavelength range, wherein x is a number greater than or equal to zeroand where (Ge) is Germanium, (In) is Indium, (Ga) is Gallium, (As) isArsenic, (P) is Phosphorus, (Cd) is Cadmium, (Se) is Selenium, (Zn) isZinc, and (S) is Sulfur.
 7. The method of claim 1, wherein at least oneof the first or second semiconductor materials is Si and at least one ofthe first or second semiconductor materials is selected from a groupconsisting of: PbS, PbSe, InSb, GaSb, CdS, CdSe, HgCdSe, HgCdS, andHgCdTe to enable photon detection in the medium or long infraredwavelength range where (Pb) is Lead, (Sb) is Antimony, (Hg) is Mercury,(S) is sulfur, (Se) is Selenium, (In) is Indium, (Ga) is Gallium, (Cd)is Cadmium, and (Te) is Tellurium.
 8. The method of claim 1, wherein atleast one of the first or second semiconductor materials is Si and atleast one of the first or second semiconductor materials is selectedfrom a group consisting of: PbSe_(x)S_(1-x), Cd_(x)Hg_(1-x)S,Pb_(x)Hg_(1-x)S, CdSe_(x)S_(1-x), As₂S_(x)Se_(5-x),GeSe_(x)S_(y)Te_((1-x-y)), CuAlS_(2-x)Se_(x), As_(x)Se_(1-x)Ge₅,AgGaSe₂—GeSe₂, and AgGaS₂—GeS₂ to enable tunable wavelengthcharacteristics, wherein x and y are numbers greater than or equal tozero and where (Cu) is Copper, (Al) is Aluminum, (As) is Arsenic, (Ge)is Germanium, (Pb) is Lead, (S) is sulfur, (Se) is Selenium, (In) isIndium, (Ga) is Gallium, (Cd) is Cadmium, (Sb) is Antimony, (Hg) isMercury and (Ag) is Silver.
 9. The method of claim 1, wherein at leastone of the first and second nanowires is fabricated at about 2 to 100nanometers in diameter and 5 to 250 nanometers in length.